1. Field of the Invention
The present invention relates to thermoplastic resin sheets and thermoplastic resin-molded articles which can generate heat by the irradiation of microwaves, and it also relates to thermoplastic resin sheets and thermoplastic resin-molded articles having electromagnetic wave shielding properties.
2. Description of the Prior Art
In recent years, many precooked foods ovens are on the market, and as a consequence, various kinds of plastic packaging materials for microwave ovens have been contrived. Among these packaging materials, there is a packaging material prepared by laminating an aluminum-deposited film to a paper and another packaging material having the function to give burns to the surfaces of foods. In the case of the aforesaid laminated packaging material, the microwaves from the microwave oven can be utilized to generate Joule heat, by which the foods can be heated more quickly.
In order to generate the Joule heat by the use of microwaves (frequency 2,450 MHz) from the microwave oven, it is necessary to form the deposited layer of the deposited film extremely thin (about 1/10 of the thickness of the deposited film which has been used in the usual packaging material). However, if it is intended form thin deposited layers, the productivity of these films deteriorates since it is difficult to control the thickness of the deposited layer to such a small thickness; thus the production cost thereof increases. In addition, since the deposited layer has neither ductility nor malleability, deep drawing cannot be applied to these deposited films. The handling of the thin deposited films is difficult, and therefore it is required to laminate each film with another base material. However, a process for this lamination is complex, depending upon the kind of base material.
On the other hand, with the prevalence of electronic equipment such as equipment for office automation, medical equipment, devices for civilian communication and computer equipment, troubles which result from electromagnetic waves radiated from such equipment are serious social problems, and shielding of the electromagnetic waves is strongly demanded.
As techniques of providing housings or the like of the electronic equipment with electromagnetic wave shielding properties, there is a process of molding a resin in which a electroconductive filler is contained at a high concentration and a process of coating the inside walls of resin-molded articles with an electroconductive coating material.
However, in the former process, it is necessary to fill the resin with a great deal of the electroconductive filler, which causes increase in the specific gravity of obtained sheets or molded articles, a deterioration in mechanical characteristics, an increase in cost, a decline in appearance and a deterioration in moldability. Furthermore, the latter process involves problems such as the deterioration in electromagnetic wave shielding properties due to the peeling of the coating films, complex production steps and low productivity. In addition, with regard to the sheets or the molded articles which contain the electroconductive filler at a high concentration or which are coated with the electroconductive coating material containing a large amount of the electroconductive filler as described above, they have no see-through, i.e., are not transparent, and hence do not permit confirmation of the contents therein. Therefore, such sheets, and molded articles cannot be applied to uses in which transparency is required.